The present invention relates to a magneto-optical recording medium. In more particular, the invention relates to magnetic characteristics of respective films of a magneto-optical recording medium comprising a transparent substrate and, supported thereon, a magneto-optical recording film comprising a perpendicular magnetic film made of an alloy of rare earth metal-transition metal formed on the substrate, and an auxiliary magnetic film having spontaneous magnetization provided in contact with the magneto-optical recording film.
Information recording systems using a magneto-optical recording medium have come into practical use and, accordingly, the development of a magneto-optical recording medium is becoming an increasingly more important technical problem which is capable of recording new information simultaneously with erasing previously recorded information, instead of erasing previously recorded information once and thereafter writing new information.
An overwrite recording system for magneto-optical recording media which is attracting the most attention at present is the light intensity modulating system, in which information is recorded by scanning a recording light intensity-modulated in the form of pulses corresponding to a "0" or a "1" of an information signal which has been converted into binary notation, while applying an external magnetic field of constant strength to the magneto-optical recording medium. Examples of magneto-optical recording media which have enabled overwrite recording of a light intensity modulation system include the one described in Japanese Patent Kokai (Laid-open) No. 62-175948 and the one described in International Publication No. WO90/02400.
A magneto-optical recording medium is a medium which records information by forming on a magnetic layer supported on a substrate inverted magnetic domains (i.e., magnetized domains) corresponding to information signals and playing back the recorded information by making use of the fact that the Kerr angle of rotation of reflected light changes in accordance with the direction of magnetization of the magnetic layer when a linear polarized light is irradiated to the magnetic layer on which the information has been recorded as mentioned above. The inverted magnetic domain is formed on the magnetic layer through either, as so-called, the light intensity modulation system or the magnetic field strength modulation system. In both systems, the magnetic layer must be brought up to a temperature near to or higher than the Curie temperature. Further, a magneto-optical recording medium is an information recording medium of the so-called erasable type in which the erasing and rewriting of information can be conducted repeatedly and also, in erasing information, the temperature of the magnetic layer must be increased to near or above the Curie temperature. The temperature increase of a magnetic layer has hitherto been performed by focusing a laser beam on the magnetic layer.
Thus, in a magneto-optical recording medium, information is recorded and erased by heating the magnetic layer to a predetermined temperature. In order to obtain a desired data transfer velocity, therefore, the Curie temperature of the magnetic layer, the heat conductivity of the part contacting to the magnetic layer, the intensity of laser beam irradiated to the magnetic layer and the linear velocity of the laser beam spot which travels on the magneto-optical recording medium must be all set in good balance.
A common magneto-optical disk of 5 inch diameter, which is in practical use thus far, is attached to a magneto-optical disk driving apparatus provided with a semiconductor laser of 30-40 mW and is rotationally driven at 2400 rpm (at a constant angular velocity). In such a disk, the Curie temperature of the magnetic layer and the heat conductivity of the part contacting the magnetic layer are so set that the recording and erasing of information can be performed under conditions which is optimum in the outermost peripheral recording region, where the linear velocity of the laser beam spot is the highest (that is, 15 m/sec). As prior art technologies relating thereto, reference may be made to Japanese Patent Kokai (Laid-open) Nos. 56-4090, 56-54070, 57-120253 and 57-169996, for example.
As described, for example, in "Wakari-yasui Hikari-disk" (For Easier Understanding of Optical Disk, published by Optronics K.K., Dec. 10, 1985), p. 52, there is already known an optical information recording medium of so-called closely adhered lamination structure formed by sticking the transparent substrates and the recording surfaces, respectively, of two magneto-optical recording single plates to each other. There is also known an optical information recording medium of so-called air sandwich structure formed by sticking the inner peripheral parts and the outer peripheral parts of the transparent substrates of two optical recording single plates respectively to each other through a spacer coated with an adhesive so as to provide an air layer between the opposing recording surfaces.
FIG. 41 shows the external magnetic field characteristics of a conventional magneto-optical recording medium having a perpendicular magnetic film supported thereon as a magneto-optical recording film and FIG. 42 shows the bit error occurrence characteristics of the above conventional magneto-optical recording medium. In FIG. 41, the ordinate axis indicates the playback signal output and the abscissa axis indicates intensity of the external magnetic field. The positive value on the abscissa axis shows the magnetic field of the recording direction and the negative value shows the magnetic field of the erasing direction. As shown in FIG. 43, the sample used was prepared by laminating successively on one side of a polycarbonate substrate 1 an SiN type first inorganic dielectric substance layer 2 higher in refractive index than the substrate 1, a TbFeCo type perpendicular magnetic film 3 as a magneto-optical recording film, a second inorganic dielectric substance layer 4 comprising the same inorganic dielectric substance as of the first inorganic dielectric substance layer 2 and an Al reflecting film 5 by continuous sputtering. As shown in FIG. 41, in this magneto-optical recording medium, the perpendicular magnetic film 3 cannot reach saturation magnetization unless an external magnetic field of at least 200 (Oe) is applied in the recording direction or in the erasing direction. Furthermore, as shown in FIG. 42, the bit occurrence rate cannot be reduced to the lowest level unless an external magnetic field of at least 200 (Oe) is applied in the recording direction.
Moreover, a magnetic torque of the sample taken out from the above magneto-optical recording medium was measured by a torque magnetometer (Torque Magnetometer: TRT-2 manufactured by Toei Kogyo Co., Ltd.) and the change in the magnetic torque with respect to the rotating angle of the external magnetic field was plotted to obtain the torque curve as shown in FIG. 39. In this case, the ranges of measurements are Y=2 dyn-cm/V, X=0.5 mV/dog and the recorder ranges are Y=0.1 V/cm, X=10 mV/cm. The measurement was conducted in the following manner. While an external magnetic field H.sub.ex of a strength specified by H.sub.c &lt;H.sub.ex &lt;H.sub.k when the magnetic domain wall coercive force at the measuring temperature is designated as H.sub.c and the vertical anisotropic magnetic field at the measuring temperature is designated as H.sub.k is being applied to the sample, the external magnetic field is rotated around the supporting axis of the sample and the magnetic torque acting on the supporting axis of the sample was measured. "0.degree." in FIG. 39 shows that the direction of the external magnetic field is perpendicular to the film face of the sample and ".theta." shows the angle of rotation of the external magnetic field H.sub.ex from the direction perpendicular to the film face.
An optical recording single plate comprises a transparent substrate and one or more layers of film containing at least a recording layer or a reflecting layer adhered to the preformat pattern-carrying surface of the substrate. The transparent substrate is formed, for example, out of a transparent ceramic material, such as glass, etc., or a transparent plastic material, such as polycarbonate, poly(methyl methacrylate), polymethylpentene, epoxy resin, photocurable resin, etc. As the adhesive agent for adhering these optical recording single plates, polymeric adhesive agents such as hot melt adhesives and epoxy adhesives are mainly used.
In optical recording media of this type, further, various investigations have been made up to now on providing a transparent thin layer of oxide, nitride and the like between the transparent substrate and the optical recording film to make it function as an optical multiple interference film or a protective film.
In order to realize the overwrite of information, the sensitivity of a magneto-optical recording film to the external magnetic field must be sufficiently high irrespective of whether the light intensity modulation system or the magnetic field strength modulation system is adopted. Of the prior art examples of the magneto-optical recording medium applicable to overwrite recording of the light intensity modulation system mentioned above, the former has an auxiliary magnetic film having spontaneous magnetization laminated onto the magneto-optical recording film which is a perpendicular magnetic film, so that it can have an enhanced external magnetic field sensitivity in recording or erasing as compared with media of a perpendicular magnetic film alone.
However, even in the case of a recording medium of such a structure, an external magnetic field of 200 (Oe) or more is necessary in recording or erasing, so that the medium cannot be regarded as having a sufficiently high external magnetic field sensitivity for practical use. Furthermore, since the magneto-optical recording film and the auxiliary magnetic film are laminated directly, the exchange coupling force working between the two films becomes considerably higher than the desired value, so that when the light intensity modulation system is used an initialization magnet of at least 5-6 (KOe) becomes necessary and further, in company therewith, the recording magnetic domain tends to disappear.
Further, in a magneto-optical recording medium, one of the most important technical problems is to increase the data transfer velocity. In order that the data transfer velocity may be improved, the linear velocity of the laser beam spot relative to the magneto-optical recording medium (that is, for example, the rotational velocity of a magneto-optical disk) must be increased. However, when the linear velocity of the laser beam spot is increased, the irradiation time of the laser beam is shortened and hence the temperature of the magnetic layer is raised with more difficulty. Consequently, a technical problem of either improving the recording sensitivity of a magneto-optical recording medium or greatly increasing the output power of a semiconductor laser must be solved.
Further, in an optical information recording system of a lamination structure, an inorganic filler is frequently incorporated into the adhesive layer to decrease the water vapor permeability of the adhesive layer and thereby to prevent the penetration of water into the medium and prevent the corrosion of the recording layer or the reflecting layer. In such cases, since the reactivity between the inorganic filler and the adhesive agent is poor, peeling tends to occur, resulting in poor long term storability of the medium.
In the case of an optical information recording medium of the type in which an oxide layer is formed by means of vapor deposition or sputtering, the oxide layer tends to become chemically unstable and liberate oxygen, and the liberated oxygen gradually diffuses into the optical recording film and oxydizes the optical recording film, leading to deterioration of information-recording, -regeneration and-erasing characteristics.
On the other hand, since the adhesiveness of a nitride layer to a transparent substrate made of synthetic resin such as polycarbonate is poor, if it is used as the undercoat layer of optical recording film peeling will occur between the transparent substrate and the undercoat layer over a long period of time so as to decrease the usefulness of the medium for data storage.
From the problems described above, optical information recording media of the prior art are not satisfactory in reliability.